This invention relates generally to variable-gain small signal amplifiers and more particularly to a linear-in-decibel gain control circuit.
In communications systems, analog receivers vary the amount of gain according to the specific receiver operation and the strength of the received signal in order to maintain a constant signal level. Variable-gain amplifiers are typically used to achieve this desired effect in the receiver. Because of the wide range of received signal strength, the variable gain amplifier must be able to vary its gain over a wide range.
So-called linear-in-decibel variable gain amplifiers are known to provide this desired level of gain control. These amplifiers take advantage of the fact that a change in forward current through a diode produces a change in voltage across the diode that is proportional to the logarithm of the current according to the well-known junction equation: VF=(kF/q) log (I.sub.F /I.sub.S). For example, a decade increase in forward current at room temperature produces a voltage increase of about 60 mV for a typical diode. Thus, variable gain amplifiers can be designed that produce a decade of gain increase for every 60 mV of voltage change across a diode.
An example of such a linear-in-decibel variable gain amplifier is shown in U.S. Pat. No. 3,736,520 entitled "Precision Variable Gain Amplifier with Linear Log-Gain versus Control-Voltage Characteristic," by Acker. The problem with this and other linear-in-decibel variable gain amplifiers is that they are complex, typically requiring one or more operational amplifiers to implement. They also suffer from accuracy and bandwidth limitations.
Accordingly, a need remains for a simple yet accurate variable amplifier which is linear-in-decibels.